Several instruments and methods for analyzing particles by measuring the phase shift of an incident laser beam scattered by particles are known. These methods consider a particle as a phase object and measure the effect of the particle on a waveform. Some of these instruments and methods conduct measurements in the so-called "bright field," others in the "dark field". A bright field configuration detects light scattered in the forward direction. A dark field configuration detects light scattered at an angle to the incident beam.
Light scattered in a forward direction by a small particle causes a phase shift and an attenuation of the incident beam dependent on the size and index of refraction of the particle. In U.S. Pat. No. 5,133,602, "Particle Path Determination System," by J. S. Batchelder et al., U.S. Pat. No. 5,061,070, "Particulate Inspection of Fluids Using Interferometric Light Measurements," by J. S. Batchelder, et al., and U.S. Pat. No. 5,037,202, "Measurements of Size and Refractive Index of Particles Using the Complex Forward-Scattered Electromagnetic Field," by J. S. Batchelder, et al., phase shift and extinction of a pair of orthogonally polarized laser beams are employed to characterize particles in the fluid. The forward direction of the scattered light is detected using an interferometer which measures the phase shift of one beam relative to another.
A different technique is employed for analyzing a particle in a dark field configuration. In U.S. Pat. No. 4,540,283 to Bachalo, for example, a dark field system is described in which two incident beams are crossed to set up an interference pattern that creates a varying electrical signal from changing fringes when a particle passes across the pattern. U.S. Pat. No. 4,764,013 to Johnston describes a dark field method that determines the phase difference between two polarization components of scattered light. Batchelder et al.s' U.S. Pat. No. 5,061,070, discussed above, also states that a dark field arrangement using a heterodyne interferometer, where a sample beam and a reference beam are provided having different wavelengths, can be used with its phase shift analysis technique.
In both bright and dark field particle detection systems, it is very important to know the dependence of the measured signal on the particle's trajectory in the vicinity of the laser beam profile.
Because most dark field optical configurations employ focused-beam geometry, it is also important to characterize the intensity distribution of the laser beam in a sampling volume or surface to be analyzed in order to determine the size of particles. Beam profile non-uniformity can cause measurement errors.
A correction system utilizing two coaxial laser beams of orthogonal polarization, varying widths, and coincident focal planes, is described by Knollenberg in U.S. Pat. No. 4,636,075. Knollenburg's design requires that the signal from a tightly focused laser beam increase above a threshold before taking the signal from the wider beam to obtain a particle size. It is therefore limited in the minimum size particles it can detect.
U.S. Pat. No. 4,854,705 to Bachalo describes another dark field detection system wherein concentric beams of different focal spot size are used to determine a particle's trajectory.
Regardless of the phase measurement technique employed in the above patents, the phase shift measurement is based on the intensity of the scattered signal and-does not provide heterodyne detection of the polarization states of polarized incident light scattered by a particle.